1. Field of the Invention
This invention relates to an information radio transmission system that performs radio transmission of digital data such as static image data.
2. Description of the Related Art
Many of the conventional radio data transmissions employ a retransmission mode. That is, when an error occurs in transmitted data, the receiving side requests the transmitting side to retransmit the data area where the error has occurred, thereby realizing reliable data transfer.
In this case, if the condition of the transmission line is bad and data has a large error volume, the volume of retransmitted data increases. Errors also occur in the retransmission of data. Therefore, it is considered that as the volume of transmitted data increases, the volume of retransmitted data exponentially increases. As the volume of retransmitted data increases, it takes time to transfer data and the time during which the transmission line is occupied becomes longer. This is not preferable. Thus, a method of controlling transmitted data in accordance with the condition of the transmission line is proposed. Since the condition of the radio transmission line varies depending on the peripheral environment and time, it is necessary to control data in accordance with the variance of the transmission state.
JP-A-11-308297 (pages 3 to 6 and FIG. 1) discloses a method of measuring the error volume in received data on the receiving side, preparing control information, sending the control information from the receiving side to the transmitting side, and setting the volume of transmitted data of the next transmission based on the control information on the transmitting side, thereby transmitting optimum data in a minimum time. Here, as a method of preparing control information, it is proposed to utilize data that is usually transmitted and received, and compare a bit string of received data with a bit string acquired by performing error correction of the received data.
JP-A-11-266229 (pages 3 to 4 and FIG. 1) discloses a method of controlling the frame rate of a dynamic image in accordance with the condition of the transmission state, and thus preventing turbulence in the dynamic image or interruption of received static image on the receiving side even when the error volume varies, thereby securing the quality of the system.
In the conventional technique of JP-A-11-308297, the error volume is measured from data that is usually communicated. When data is received, error correction of the received data is performed. The resulting error-corrected data is compared with the data before the error correction, thus measuring the error volume. However, when an error exceeding the error correcting ability occurs, error correction cannot be performed, and the receiving side cannot acquire the original data and cannot measure the error volume accurately. Therefore, there is a problem that it is difficult to control transmitted data in accordance with the condition of the transmission line.
The conventional technique of JP-A-11-266229 proposes a method of controlling the frame rate on the basis of the error volume when a dynamic image is used as transmitted data. However, it proposes no method of controlling transmitted data in accordance with the condition of the transmission line when a static image is used as transmitted data. There is a problem that the image quality may be degraded if the volume of transmitted data is simply controlled.